1. Field of the Invention
This invention relates generally to monolithic microwave/millimeter wave integrated circuits (MMICs) and, more particularly, to a MMIC compatible bandpass filter.
2. Discussion
Various applications of monolithic microwave/millimeter wave integrated circuits, or MMIC devices, require integration with a bandpass filter which passes on signal components in a specified frequency band (the pass band) while substantially attenuating signal components not in the pass band. These out-of-band signals are rejected and are not passed on to other circuit elements. To be effective, such a filter must pass on inband signal components with an absolute minimum amount of loss and, at the same time, must be small enough in size to be suitable for integration with MMIC devices on the same chip.
Currently available MMIC compatible filters are usually based on either lumped constant (LE) circuits, in which circuit elements include chip capacitors and spiral inductors, or parallel coupled line (PCL) geometries wherein half-wavelength circuit elements (resonators) are utilized. For example, FIG. 1 shows a typical conventional (PCL) microwave filter 10 having parallel coupled line geometry. Filter 10 includes a dielectric substrate 12 having on a top surface thereof a printed microstrip circuit element 14 consisting of a series of parallel conductive paths which individually form half-wavelength resonators. The substrate 12 further includes, on the surface thereof opposite circuit 14, a solid printed circuit layer 16 which completely covers that surface of substrate 12 and which is positioned against and electrically coupled to a conductive ground plane 18.
Ground plane 18 is generally U-shaped in cross section for receiving the Gallium Arsenide (GaAs) substrate 12 fitting therein. As the millimeter or microwaves pass through the filter they are substantially confined between printed circuit element 14 and ground plane 18. Passband waves enter the input port 13, are coupled through resonators 14 and then exit through output port 15. The out-of-band signal components are attenuated or rejected.
Both PCL and LE types of filters, however, exhibit high passband loss due to the low unloaded Q (15-25) achievable with thin substrate microstrip and miniature lumped constant circuit elements. The PCL type of filters are generally limited to bandwidths of up to 30 percent and their rejection characteristics are adversely affected by the dispersive nature of the inhomogeneous microstrip medium as well as by circuit related factors such as tight couplings. The LE type of filters have poor high frequency rejection due to component self-resonances and require a multilayer deposition process which adds to the overall cost of the device. The LE type of MMIC filters are even lossier than their conventional PCL type counterparts due to the very low Q of spiral inductors.
There is, therefore, a need for a MMIC compatible bandpass filter which is virtually loss free and which can be easily integrated with MMIC devices. It is desirable that such a filter be small and low cost and have a low loss passband of up to an octave (e.g., 8 to 15 GHz).